When shipping or storing many types of materials, particularly electronic and semiconductor components, it is desirable to know whether those components have been exposed to a particular level of humidity of the surrounding atmosphere, which humidity could cause damage to those components. For instance, electronic components can be damaged by exposure to an atmosphere having a humidity exceeding a specific threshold level, even when the threshold level is low and the exposure is only for short period of time.
To address the need for the detection of humidity levels within shipping or storage containers, humidity indicators have been developed. There are generally two types of humidity indicators. One of these humidity indicators reversibly changes color upon exposure to particular humidity levels. Such reversible humidity indicators typically utilize cobalt chloride as the humidity indicator material impregnated on blotting paper. Military specification MIL-I-8835A describes the details of construction of such indicators and performance requirements. The indicators change color when exposed to predetermined levels of humidity and return to its original color when the humidity level drops below that predetermined level. Reversible humidity indicators are used to indicate the current condition of a desiccant and/or the current humidity level within the storage container.
The second type of humidity indicator is an irreversible humidity indicator. These indicators are designed to detect a predetermined level of humidity and provide a visual indication of whether components stored in the containers have been exposed to that predetermined level of humidity. The irreversible humidity indicator can therefore provide an indication when the humidity reaches the predetermined level even for short periods of time and even if the level of humidity drops below that predetermined level when the components are checked at a later time. Large changes in humidity levels sometimes occur where storage containers are used in relatively warm climates and when the moisture level in the air rises and falls dramatically depending upon the temperature of the surrounding air. Under these conditions, a reversible humidity indicator might fail to indicate the temporary presence of high humidity within a storage container even though such high humidity may be sufficient to cause damage to the components present in the storage container. Irreversible humidity indicator cards are described in patents U.S. Pat. Nos. 6,877,457 and 6,698,378, for example.
The advent of surface mount devices (SMDs) introduced a new class of quality and reliability concerns regarding package damage “cracks and delamination” from the solder reflow process and the effect of moisture within these areas. Moisture from atmospheric humidity enters permeable packaging materials by diffusion. Assembly processes used to solder SMD packages to printed circuit boards (PCBs) expose the entire package body to temperatures higher than 200° C. During solder reflow, the combination of rapid moisture expansion, materials mismatch, and material interface degradation can result in package cracking and/or delamination of critical interfaces within the package. The presence of moisture within these cracks and delaminations may have deleterious effects on the SMDs. To address this concern, the Electronics Industries Alliance/Joint Electron Device engineering Council (JEDEC) has developed a Joint Industry Standard directed to the “Handling, Packaging, and Use of Moisture/Reflow Sensitive Surface Mount Device, IPC/JEDEC J-STD-033B October 2005. This document describes the standardized levels of floor life exposure for moisture/reflow-sensitive SMD packages along with the handling, packing and shipping requirements necessary to avoid moisture/reflow-related failures.
In this standard, a humidity indicator card is defined as “[a] card on which a moisture-sensitive chemical is applied such that it will make a significant, perceptible change in color (hue), typically from blue (dry) to pink (wet) when the indicated relative humidity is exceeded.” The standard further indicates that the humidity indicator card is normally packed inside a moisture-barrier bag, along with a desiccant and the SMDs, to aid in determining the level of moisture to which the moisture-sensitive devices have been subjected. Annex 1 of the standard describes a test to objectively assess the color change, and specifies minimum hue changes for humidity indicator cards in order to be suitable for SMD dry packing. The standard requires a humidity indicator card indicating humidity levels of relative humidity (RH) of 5% RH, 10% RH and 60% RH with an accuracy of +/−2% RH.
In the past, many humidity indicator cards for use in the electronic industry have used cobalt dichloride impregnated on blotting paper as the humidity indicating substance. These indicators change their color from blue (dry) to pink (wet) after exposure to certain levels of humidity. Cobalt dibromide has also been used a replacement to cobalt dichloride. However, humidity indicator cards based on cobalt dichloride and cobalt dibromide contain considerable amounts of heavy metals and halide anions.
Other humidity indicator card systems have been suggested based on copper chloride and synergistic salts (e.g. EP 1 200 819 A0, also published as WO 01/09601 A1). All formulations described in WO 01/09601 A1 contain copper chloride and up to 30 times the amount of copper chloride in synergistic salts, and thus exhibit a considerable amount of compounds containing halogen and heavy metals in the humidity indicator card.
In EP 1 293 773 A2, another humidity indicator card system has been suggested on the basis of certain pH indicator dyes, selected from the group of m-cresol purple, thymol blue, tropaeolin 00 and p-xylenol blue, and an excess of a zinc salt (e.g. zinc chloride) of at least 1:5, preferably 1:20 and most preferably 1:30. Humidity indicator cards based on this disclosure contain large amounts of anions as well as heavy metals due to the excess of zinc salts.
Yet another humidity indicator card system was suggested based in US 2005/0106735 A1 on pH indicator dyes, color enhancing additives (acids and bases) and hygroscopic salts. Hygroscopic salts like magnesium chloride and sodium chloride are needed to practice this invention, thus leading to a high amount of anions in the humidity indicator card. Neutral red is described herein in combination with sodium hydroxide and magnesium chloride to give indicator spots that change color from forest green (dry) to rusty brown when being moisturized.
A humidity indicator card based on formulations of a combination of pH indicators, such as neutral red, crystal violett, brilliant green, methyl violet, methyl orange and organic acid, such as acetic acid or citric acid, is suggested in US 2007/0157702 A1. It was found that this system shows no sharp color transition from a first color to a different second color when a specified humidity level is achieved. Furthermore, it was found that the system can not be adjusted to change color at different humidity levels.
In JP 2007-198828 A, a moisture indicator based on a bilayer constitution, layered sequentially with a pH indicator and a water-soluble salt containing layer is introduced. As pH indicators bromocresol green, methyl red, chlorophenol red, bromocresol purple, neutral red, phenol red, cresol red, alpha naphthol phthalein, curcumin, metacresol purple, ethylbis(2,4 dinitrophenyl)acetate, thymol blue, para-xylenol blue, phenolphthalein, alkali blue, alizarin yellow, phenol purple, phenol blue, and bromophenol blue are described. Further on the optional addition of glycerin as a moisturizer is mentioned.
An irreversible humidity indicator card system was suggested based on a deliquescent salt combined with a water-soluble dye and optionally wetting agents, penetrating agents, or spreading agents etc. in U.S. Pat. No. 2,214,354. These agents facilitate the absorption of water and diffusion of the used paper. The addition of dyes for an enhancement of the color change of a humidity indicator is known and described in the JP 61-154986.
Surprisingly, it has been found that the addition of certain color change enhancing agents to the impregnating solution to make a humidity indicator spot allows a formulation based on organic dyes without using heavy metals to maintain the nature of the color change and its accuracy known by heavy metal containing devices.
Only recently, the semiconductor and electronic industries have adopted specifications and standards for the restriction of amounts of certain elements present in their products. For example, these industries restrict the amounts of halogen (i.e. of compounds containing F, Cl, Br and/or I), phosphorous and certain heavy metals irrespective of the chemical nature of their presence. These restrictions apply to semiconductor and electronic products as well as to components and packaging materials, including humidity indicating devices.
Thus, there is a need of humidity indicating devices containing a reduced amount of halogens and free of heavy metals. However, accuracy, readability and reliability of conventional humidity indicating devices should not by deteriorated by the omission of heavy metals and halogen-containing compounds.
It is an object of the present invention to provide a composition for a humidity indicating device containing no halogen compounds and no heavy metals and having the same or improved accuracy, readability and reliability as compositions for conventional humidity indicating devices.
It is a second object of the present invention to provide a humidity indicating device containing only low amounts of halogen compounds and heavy metals and having the same or improved accuracy, readability and reliability as conventional humidity indicating devices.
It is a third object of the present invention to provide a process for preparing said humidity indicating device.